Lagerwall and Clark described the surface-stabilized ferroelectric liquid crystal (SSFLC) effect and its application to electro-optic shutters and display devices (U.S. Pat. Nos. 4,367,924 and 4,563,059). SSFLC devices can display electro-optic effects with very fast (sub-microsecond) switching speeds.
Tilted smectic liquid crystal phases particularly smectic C phases composed of chiral, nonracemic molecules possess a spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Myer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions.
In SSFLC cells, the FLC is aligned between transparent electrodes in the so called "bookshelf" alignment in which the smectic layers are substantially perpendicular to the electrodes and the long axis of the FLC molecules are parallel to the electrodes. In this configuration, the natural helix typically formed in the ferroelectric phase is suppressed by surface interactions in the cell. Suppression of the helix results in a bistable cell in which the optic axis of the cell can be rotated in the plane of the electrodes by 2.THETA., where .THETA. is the tilt angle, by changing the sign of the applied driving voltage. Tilt angle is an intrinsic property of a FLC material. This switching of rotation of the optic axis can be employed for light modulation. Within a large range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (P.sub.s), and directly proportional to orientational viscosity. Fast switching speeds are associated with FLC phases which possess high polarization density and low orientational viscosity.
In order to suppress the helix, the SSFLC cell thickness (d) must be comparable to or smaller than the magnitude of the pitch of the helix in the ferroelectric phase. Thus, for applications in the visible in which cell thicknesses of 0.5-6 .mu.m are most useful (assuming a birefringence of 0.15-0.3), the SSFLC natural ferroelectric phase helix pitch in the FLC should be longer than 0.5-10 .mu.m.
Electro-optic effects in FLC cells in which the helix in the smectic C* phase is not suppressed by surface-stabilization have also been described. The distorted helix ferroelectric (DHF) effect, described for example in Ostovski et al., Advances in Liquid Crystal Research and Applications, Oxford/Budapest. (1980) page 469 and in Funfschilling and Schadt (1989) J. Appl. Phys. 66(8):3877-3882), is observed in FLCs aligned between electrode plates in which the natural helix pitch in the smectic C* (or other chiral tilted smectic ferroelectric) phase is sufficiently tight, i.e., shorter than the FLC cell thickness (d), so that the helix is not suppressed. DHFLC electro-optic devices have an FLC aligned between electrode plates. Most typically the FLC is planar aligned and in the "bookshelf" geometry. A driving voltage is applied to the electrodes to generate an electric field across the FLC layer. Unlike, SSFLC devices, the natural helix of the aligned chiral smectic phase is present in the aligned FLC material in the DHF device. The helix forms parallel to the plates and perpendicular to the smectic layers. The magnitude of the pitch of the helix is the distance along the helix axis for one full turn of the helix and the sign of the pitch (+ or -) represents the direction of twist of the helix. The term "tight" pitch, which can be a positive or negative value, is associated with shorter axial lengths for one full turn of the helix. The term "pitch" as used herein refers to the magnitude of the pitch; the terms "sign of the pitch" or "twist" refer to the direction of twist of the helix.
When the magnitude of the ferroelectric C* phase, helical pitch is comparable to the wavelength of visible light, a striped pattern appears in the device and in effect a diffraction grating is formed. If the magnitude of the pitch is less than the wavelength of light (and preferably less than 1/2 .lambda. of light) light diffraction is minimized and the apparent refractive index of the FLC is the average over many director orientation of the helix. In the field-free state with zero applied electric field and with no surface stabilization, the C* helix is in its natural state. The molecular director, n, makes an angle, .THETA., with the layer normal. In the field-free (E=0) state, due to the presence of the helix, averaging occurs and the apparent optic axis of the DHFLC coincides with the helix axis.
If the voltage applied across the FLC layer is above a certain critical level E.sub.c, the helix is completely unwound forming two distinct optical states, as in an SSFLC device. Application of a voltage below E.sub.c deforms the helix, generating an effective rotation of the optic axis of the DHFLC. The orientation of the optic axis of the DHFLC layer can be changed in a continuous fashion proportional to the applied electric field changing the optical anisotropy of the FLC. DHF cells display rotation of their optic axis that is dependent on the magnitude of the applied electric field and also exhibit a change in apparent birefringence (.DELTA.n) as a function of the magnitude of the applied electric field.
The maximum field-induced angle of rotation of the optic axis of the DHFLC is .THETA., the tilt angle of the material. A maximum field induced optic axis rotation of 2.THETA. can be obtained by application of a .+-. voltage step, .+-.E.sub.max, where E.sub.max is the minimum voltage required to obtain a rotation of .THETA. and the magnitude of E.sub.max is less than E.sub.c.
DHF-effect cells typically exhibit significantly lower apparent refractive index than SSFLC cells due to the averaging noted above. Thus, for a given desired optical retardation, DHF cells are typically thicker than comparable SSFLC cells. Birefringence for DHFLC cells typically ranges from about 0.06 to 0.13, about 1/2 that of SSFLC cells. DHFLC waveplates are as a consequence, typically, thicker than comparable SSFLC waveplates. High birefringence materials are thus useful in DHF applications to minimize cell thicknesses.
E.sub.c is inversely proportional to the spontaneous polarization of the FLC and the ferroelectric phase pitch, having the relationship: ##EQU1## Thus, the higher the spontaneous polarization and longer the pitch, the lower the voltage necessary to control the effect. Response time (.tau.) for the DHFLC cell is a function of pitch, tilt angle and viscosity: ##EQU2## where .gamma. is the orientational viscosity and .THETA. is the tilt angle. Increasing P.sub.s lowers the threshold voltage, but does not increase the speed, while tightening the pitch increases both the speed and E.sub.c. By increasing both P.sub.s and decreasing p, the response speed can be significantly increased while maintaining a low threshold voltage. Also decreasing the viscosity improves the response time.
Contrast ratio of a device is defined as the ratio of the transmitted light in an ON (maximal white light transmitted through the device) and an OFF (minimal white light transmitted through the device) state. Maximum contrast is obtained when the voltage step applied across the cell rotates the optic axis by a total of 45.degree. between OFF and ON states. Maximum transmission in the ON state can be limited if the total optic axis rotation is less than 45.degree., as in FLC's which have tilt angles less than 22.5.degree.. Most often, however, contrast is limited by light leaking through in the OFF state, a function of the quality of cell alignment. Minimal OFF state transmission in both SSFLC and DHFLC requires good uniform alignment.
It is well-known in the art that improved alignment and contrast ratio in SSFLC cells can be facilitated by an FLC having a long pitch N* phase at higher temperatures to the ferroelectric tilted chiral smectic phase (see for example WO 87/06021). To facilitate alignment in SSFLCs, N* pitch should be at least equal to d, and preferably 4d or more. It is also well-known in the art for the preparation of SSFLC cells that cell alignment is further facilitated by the presence in the FLC of a smectic A phase intermediate in temperature between the chiral tilted smectic ferroelectric phase and the N* phase. SSFLC cells, as noted above, however, are also considered in the art to require relatively long pitch (typically longer than d and preferably longer than 4d) in their ferroelectric phase.
Methods analogous to those that had been successful in improving the alignment and contrast of SSFLC cells can be employed to improve the alignment and contrast in DHFLC cells. Wand et al. U.S. Ser. No. 832,414, filed Feb. 7, 1992, which is incorporated in its entirety herein by reference, reports that compositions having a tight pitch ferroelectric phase, e.g., a smectic C* phase, and a long pitch N* phase at higher temperatures can be aligned using methods such as those described in WO 87/06021. These methods combine cell surface treatment (i.e., alignment layers) with cooling of the FLC in contact with the treated surfaces of the cell plates from the nematic phase to the ferroelectric phase. Good FLC alignment and high DHFLC cell contrast result. It was also found by Wand et al. U.S. Ser. No. 832,414 that the presence of an orthogonal smectic phase, such as a smectic A phase, intermediate in temperature between the nematic and the ferroelectric phases further facilitates good alignment and the generation of high contrast DHFLC cells.
A basic requirement for application of ferroelectric liquid crystals in electro-optical devices is the availability of chemically stable liquid crystal compounds or mixtures which exhibit ferroelectric phases (chiral smectic C) over a substantial temperature range about room temperature. In some cases, the ferroelectric liquid crystal compound itself will possess an enantiotropic or monotropic ferroelectric (chiral smectic C*) liquid crystal phase. Ferroelectric liquid crystal mixtures possessing smectic C* phases with useful temperature ranges can also be obtained by admixture of chiral, nonracemic compounds, designated ferroelectric liquid crystal dopants into liquid crystal host material (which may or may not be composed of chiral molecules). Addition of the dopant can affect the ferroelectric polarization density and/or the viscosity of the C* phase and thereby affect the switching speed. Desirable FLC dopants are molecules which impart high ferroelectric polarization density to an FLC material without significantly increasing the orientational viscosity of the mixture. The components of FLC mixtures can also be adjusted to vary phase transition temperatures or to introduce desired LC phases. The components of FLC mixtures can also be adjusted to vary N* pitch and C* pitch.
Thermotropic liquid crystal molecules typically possess structures which combine a generally linear and generally rigid liquid crystal core coupled with two relatively "floppy" tails (see Demus et al. (1974) Flussige Kristalle In Tabellen, VEB Deutscher Verlag fur Grundstoffindustrie, Lebzig for a compilation of the molecular structures of LC molecules). FLC dopants typically possess rigid LC cores and at least one flexible tail. FLC materials have been prepared by the introduction of a stereocenter into one (or both) of the tails, thus introducing chirality.
In bistable SSFLC applications, large P.sub.s (spontaneous polarization density), fast rise time, low orientational viscosity, long N* pitch and long C* pitch are desirable. Large P.sub.s, fast rise time, and low orientational viscosity all relate to the switching speed upon application of an optimal field. The N* and C* pitch are both manifestations of the chirality of the liquid crystal material and are intrinsic properties of FLC components. Although both are helices formed in the liquid crystal, they propagate in different directions and bring different complications to a FLC light modulator. The N* helix, in a surface stabilized FLC with planar geometry, runs perpendicular to the substrates, whereas in the same FLC, the C* helix runs parallel to the substrate. As noted above, the N* helical repeat length or pitch, measured at the N.fwdarw.A or N.fwdarw.C transition, should be more than four times the width of the cell to give preferred consistent alignment of the FLC (Uchida, T. et al. (1989) Liquid Crystals 5:1127).
In DHFLC applications large Ps, fast rise time, low orientational viscosity also are desirable and a very tight C* pitch is required. For good alignment, DHFLC materials preferably combine long N* pitch with the very tight C* pitch.